# Tag Info

### Effective field theory odd dimension operators

This paper (and its references such as this one in Appendix A) contain a general proof. It shows that if you have an operator that violates $B$ of an amount $\Delta B$ and $L$ of an amount $\Delta L$, ...

### Can a (conservative) four-force be derived from a scalar potential?

Your calculation is consistent, a bit misleading. You should view it as a constraint on the possibilities of $\Phi$ since the force $f$ must be spacelike. This is the case here since $\Phi$ has only ...
1 vote

### How is there energy and charge in the universe?

Well, indeed energy is not conserved in curved spacetime. It's only locally conserved. Unless one takes into account the gravitional self energy. Gravitational Pseudo-Tensor of Energy-Momentum One ...
1 vote

### How is there energy and charge in the universe?

Experimentally, we see processes like pair production that produce charged particles but conserve charge by producing equal positive and negative charge. We presume that the charged particles we see ...

### How does conservation of energy differ from conservation of momentum in this problem?

One way of thinking about this is that energy always has to do with "state changes". "State" is all of the measureable quantities that describe an object or system of objects. ...

### How does conservation of energy differ from conservation of momentum in this problem?

In an "inelastic collision" the KE isn't conserved (due to internal friction), so that approach would lead to a wrong result. A collision where two bodies stick together is usually called a &...
1 vote

### How to find distance between colliding objects?

It depends on the material of the two balls. A hard material such as glass may allow the balls to gain $\frac{mv}{m+M}$ velocity quickly and a soft material may allow the balls to gain that velocity ...

### Singularities/infinities of continuity equation in polar coordinate

Entering your "utmost general solution" on the left-hand side does not yield a zero, but $$\frac{f(r-vt)\left|V\right|}{r^2}\,.$$ (Assuming constant velocity. With non-constant velocity we ...

### The divergence of the stress-energy tensor vanishes; is this statement sufficient to derive the Einstein field equations?

No because this tells you nothing about the geometry of space. Since the divergence of the stress-energy tensor vanishes for any geometry, it can't provide any information about the spacetime. ...

### Momentum and energy conservation and preconditions

Short answer to your question: energy conservation does require both considering the falling body and the earth, as the gravitational potential energy is shared among the earth and the falling body (...

### Momentum and energy conservation and preconditions

Momentum is conserved in a system if there are no external forces on the system. Likewise, angular momentum is conserved in a system if there are no external torques on the system. Energy is conserved ...
1 vote
Accepted

### Momentum and energy conservation and preconditions

what the preconditions for applying momentum and energy conservation in mechanics are? We write the laws of physics that describe a system in the system’s Lagrangian. If the Lagrangian is not an ...
1 vote
Accepted

### Superposition principle in classical collision theory

It might help to draw a free-body diagram of the particle as it collides with the wall. Presuming a smooth wall, the force by the wall on the particle is perpendicular to the wall's surface. So, apply ...

### What is the symmetry which is responsible for conservation of mass?

It seems that space-time translations leads to an approximate conservation for mass, in the non-relativistic limit. Let's see how: For a relativistic perfect fluid, we have the following energy-...
1 vote

### Can we deduce the conservation of mass in non-relativist physics or is it just an experimental fact?

Fundamentally, we don't deduce things in physics. We experiment and observe, and adjust our mathematical models accordingly. All deduction from math is suspect when applied to physics. Mathematical ...
1 vote

### The effect of changing $g$ for an elastic bouncing ball

Energy is conserved. For coefficients of restitution less than 1 (the normal case), some of the ball's "bouncing energy" is turned into heat upon each bounce, and the ball slowly comes to ...
1 vote

### Why is the center of mass of this system moving?

As far as I understand the man and the ladder both have equal mass. Assuming the man can climb up the ladder without exciting movement in the other mass, the center of mass will shift. This is not a ...

### Do quantum measurements violate conservation laws?

No. Remember that, even before quantum mechanics, conservation laws apply only to closed systems. When a "quantum measurement" is performed, the previously-closed system becomes an open ...

### Do quantum measurements violate conservation laws?

Do quantum measurements violate conservation laws? The theory of quantum mechanics that is used to model and predict the results of experiments at the level of particle and nuclear physics, is based ...
1 vote
Accepted

### Law of conservation of momentum and interference

On screen 4 you will still see "interference", as with photons each particle/wave will determine its own path (per Dirac, Feynman and QM) and all electrons going to the right have the option ...
Accepted

### Why do "good" quantum states remain stationary under perturbation?

Good quantum states are those that happen to diagonalize the perturbation. The effect of the perturbation on the original Hamiltonian is then merely an increase in the diagonal entries, which are the ...
1 vote

### Lt. Joe Kenda's expertise(?) in fundamental physics

As to the observation that people tends to slump forward when they've been shot. When a person is standing upright most of of the weight of that person is carried by the forefeet. You can try that as ...
1 vote
Accepted

### Lt. Joe Kenda's expertise(?) in fundamental physics

While bullets travel fast, they are extremely light compared to the mass of the human body, so they barely cause any backward motion on the victim. Let's do the math. Consider a $80\text{ kg}$ target ...

### Conservation theorem for cyclic coordinates in the Lagrangian

In Goldstein, it has been proved that if $q_j$ is such that if it is changed by $\epsilon$, then all other coordinates also gets changed by the same amount in the direction of $q_j$ (whole system gets ...

### Conservation theorem for cyclic coordinates in the Lagrangian

First, to clear the confusion about translation, when one shifts one of the coordinates of the system, you are translating the entire system. For example, consider a particle placed on a 2D plane with ...
At the highest point, we have $v_x = v sin \alpha$ and $v_y = 0$ for each particle, although their $v_x$ are in opposite direction. [$v_y$ = 0 at the top of a projectile] Hence, $p_y = 0$ and \$p_x ...